The present invention relates to fluorescent lamps and, more particularly, to fluorescent lamps employing near ultraviolet radiation sources together with phosphor material which is index matched to an outer envelope.
In conventional fluorescent lamps, most of the visible light output originates in a thin layer of phosphor which is disposed immediately adjacent to a mercury discharge. This fact is a consequence of the high optical absorption coefficients that most phosphors exhibit at the 254 nanometer wavelength of mercury resonance radiation in the far ultraviolet spectral region. Since much of this light is directed back into the lamp be scattering, useful light output is lost by multiple scattering within the outer portion of the phosphor material and by absorption at the ends of the lamp.
It would be desirable to minimize this visible light scattering loss by optimizing the scattering characteristics of the phosphor for escape of the visible length radiation from the lamp. However, in conventional lamps where the excitation occurs as a result of the mercury resonance radiation at 254 nanometers, it is not possible to imbed phosphors in a suitable optimizing matrix since no low melting point material which is transparent to this short wavelength radiation is available.
However, in U.S. patent application Ser. No. 288,822, filed July 31, 1981, there is disclosed a compact fluorescent lamp with copper arc excitation. The lamp disclosed therein is an efficient producer of near ultraviolet radiation. For example, if a copper halide is used in the ionizing discharge medium, ultraviolet radiation at 324.7 and 327.4 nanometers is produced. It is the use of a practical, near ultraviolet source for phosphor excitation which makes possible the consideration of employing materials, particularly plastic materials and low melting point glasses, in which a phosphor may be embedded to optimize optical properties. More particularly, the present invention discloses a fluorescent lamp in which the phosphor and embedding matrix material comprises substances which are matched to one another with respect to their indices of refraction or in which an optically homogeneous solution of phosphor and matrix is employed.
It is noted, however, that the use of phosphor materials embedded in a light-transmissive matrix which is matched to the phosphor in terms of their respective indices of refraction has been employed in the past in significantly different applications. In particular, the use of such phosphors in index matched plastic material has been proposed as a substitute for certain crystal X-ray detecting phosphor bodies. However, consideration of such materials in fluorescent lamps was not possible until the development of the fluorescent lamp described in above-mentioned application Ser. No. 288,822, submitted in behalf of the same inventor as herein, and assigned to the same assignee as herein. Accordingly, the above-described patent application is hereby incorporated herein by reference.
In short, since conventional fluorescent lamps employ a mercury resonance radiation at the far ultraviolet region of the sprctrum, around 254 nanometers, it has not been possible to dispose the phosphor within a matrix medium since no low melting point material transparent to such short wavelength radiation was available. Furthermore, even though phosphors have been index matched to matrix media in the past, for the purposes of applications such as X-ray detection, index matching of fluorescent lamp phosphors was not generally thought to be available because of the lack of a suitable matrix material which was transparent to far ultraviolet radiation. However, a newly-developed, practical fluorescent lamp producing ultraviolet radiation in the near ultraviolet range has now made it possible to consider embedding phosphor materials in a matrix medium which is transparent to the near ultraviolet radiation.